Channel Emulation for Improved MIMO Product Design

MIMO technology will be the foundation for the next generation of Wi-Fi products. Whereas existing 802.11 devices use a single transmitter and single receiver, known as SISO, next generation 802.11n devices will use MIMO to leverage multiple transmit and receive antennas, delivering higher throughput (more megabits per second) and greater range (more distance between AP and client). The propagation path impairments that can degrade the performance of existing SISO devices are leveraged in MIMO to “tune” transmissions, minimize errors and improve overall performance. As a result, MIMO technology will enable Wi-Fi products to support emerging voice, video and data applications that existing SISO products are unable to handle.

Figure 1. Figure Depicting MIMO Cross
Channel Effects.

Effective testing methodologies are becoming increasingly important for Wi-Fi
chipset vendors and system manufacturers due to the growing complexity of next-generation
MIMO systems and the potential reward of being early-to-market with new technology.
This article will discuss the new breed of channel emulators, and explain why
channel emulation is crucial to delivering maximum performance in MIMO-based products.

MIMO Compared to SISO

SISO technology uses a single RF transmitter and receiver in Wi-Fi devices such as APs and client cards; MIMO implies multiple transmitters and receivers. The 802.11n task group has defined MIMO configurations that range from two transmitters and two receivers (2X2) up to four transmitters and four receivers (4X4) per AP/client. Many of the proprietary MIMO products currently available are based on 2X3 technology (two transmitters and three receivers).

In the wireless realm, the number of channels relates to the number of paths an RF signal can take between the input and output mechanisms. A wireless network based on SISO technology has one channel in each direction from the transmit antenna to the receive antenna. In comparison, a 4X4 MIMO configuration has sixteen channels in each direction because the output of each of the four transmitters can be received at all four receivers. (See Figure 1). Multipath, described below, can occur for each of these channels.

Multipath and Wi-Fi Performance

Figure 2. Multipath creates multiple
versions of the signal by virtue of reflections from walls, floors, ceilings,
furniture and people. The reflections add together in the air presenting
a challenge to the receiver of separating out the original signal. Until
now multipath was a problem that limited operating range. Now MIMO radios
actually use multipath to achieve gains in operating range.

Imagine an RF transmission filling a room and bouncing off various obstacles,
such as the ceiling, walls and furniture. These reflections cause transmissions
to take different paths before arriving at their destination  which is known
as multipath. Each of the reflected signals will arrive at the receiver(s) at
different times and with different strengths. Typically, each path in the channel
is characterized by a delay in time (some paths have more reflections and thus
take longer), by a change in amplitude (some paths cause greater signal loss than
others), and other, more subtle factors (such as the angle of arrival, angle of
departure, or angular spread).

Multipath is difficult to predict and control because it is affected by everything from building construction to the movement of people. Most SISO-based Wi-Fi products are degraded by multipath, which affects signal quality and robustness of the connection. However, MIMO technology leverages the existence of multipath between the multiple transmitters and receivers to improve network performance. For example, spatial multiplexing can double or triple a system’s throughput by transmitting different data over multiple antennas, rather than using one antenna to transmit data over several timeslots. Multipath causes these transmissions to arrive at the destination at slightly different times and at slightly different amplitudes and phases. MIMO algorithms enable the receivers to use these differences to distinguish between the signals and process the intended transmissions.

Mathematical Models for Determining Multipath

Since wireless performance depends upon so many variables, each environment is unique and no wireless system will perform the exact same way in all real-world situations. To provide wireless manufacturers with a baseline for characterizing and testing multipath behavior, the IEEE task group working on 802.11n, the next generation Wi-Fi, has defined six standard channel models. Each of these models (referred to as models A-F) details a mathematical formula that describes the multiple paths that RF transmissions take during communication between AP and client, and the effect of these paths on the signal. Many silicon developers, communications researchers and MIMO product development teams are now using models A-F in simulations to determine optimum system design (e.g. the appropriate number of transmitters, receivers, power, etc. of the various MIMO implementations).

Channel Emulation for Testing Multipath

A MIMO test platform must be able to emulate the multipath experienced by real
world APs and client devices. A channel emulator subjects RF signals to these
reflections and measures the effects that would occur in the real world. This
is achieved by:  Digitizing an RF signal from a wireless device (AP or client);
 Impairing the digitized signal by injecting delay, amplitude variation,
and other perturbations based on one of the 802.11n task group’s six mathematical
models;  Regenerating the impaired signal into an RF signal that can be read
by an AP or client.

Channel emulators can be characterized by two fundamental characteristics: the number of channels and the number of taps. Although channel emulators use many other factors to govern signal performance, channels and taps are the most simple and critical functions. The number of channels governs how many paths must be emulated. Each channel must be subjected to different statistic impairments to emulate obstacles and reflections that would be present in real-world environments. Taps dictate how much control the user has over signal delay, fading and other factors. Generally, more taps are preferred because they allow greater control over the channel. The models defined by the TGn task group, for example, require 18 taps per channel.

Channel emulators must be bi-directional  independent forward and reverse paths  to account for products that are tuned in each direction with technologies such as beam forming. Beam forming products already exist in the market and the technology is likely to be included in the future 802.11n standard. In a SISO configuration with one channel in each direction, a channel emulator with two channels would be sufficient. A MIMO channel emulator, however, must have multiple RF inputs, multiple outputs and up to 16 channels (for a 4x4 system) in each direction for a total of 32 channels. The channel emulation model will vary the signal so that the effect of transmitter 1 on each of the four receivers will be different. The impairments are introduced by the channel emulator’s tap settings. For example, transmitter 1 may be “heard loudly” by receiver 4 at one point, and “heard faintly” at another point.

As MIMO technology moves out of research laboratories and into mainstream products, ease-of-use becomes a critical requirement for today’s channel emulator users. Programmability and standard configurations (inputs, outputs and channel models) should be integrated into the platform to enable a variety of engineers to use the system and allow a company to maximize its equipment investment. MIMO channel emulators can be used across functional organizations and development teams for a variety of testing, including:
 Test MIMO algorithms and debug errors;
 Optimize the performance of existing/future Wi-Fi devices in MIMO environments;
 Streamline QA processes for new MIMO products;
 Perform competitive performance benchmarks;
 Test interoperability between MIMO implementations from multiple vendors;
 Define industry-wide rate vs. range test suites for future 802.11n products.

Conclusion

MIMO technology is the foundation for the next generation of 802.11n Wi-Fi products. By leveraging multiple transmit and receive antennas, MIMO products can deliver greater wireless throughput and range to support emerging voice, video and data applications. At the heart of MIMO technology is the concept of multipath—the reflection of RF signals in a physical environment. Whereas multipath can degrade the performance of existing 802.11 SISO devices, MIMO products take advantage of these reflections to "tune" transmissions, minimize errors and improve overall performance.

Channel emulation is a critical element for repeatable testing of MIMO technology. By subjecting RF signals to real-world obstructions and reflection conditions, an effective channel emulation device can accurately create multipath behavior and measure its effect on system performance. The new breed of MIMO channel emulators are a cost-effective replacement for traditional Wi-Fi field tests, which are time consuming and lack repeatability in multipath environments.

Jeff Abramowitz, is the vice president of marketing for Azimuth Systems. Jeff
has 20 years of experience in the wireless industry; the last 11 years have been
marketing Wi-Fi systems and semiconductors. Graham Celine, is the senior director
of marketing. Graham has 15 years of high tech experience, joining Azimuth after
a 13-year career focused on data networking. CDMA algorithm and architecture development.